Totally < ω-computably enumerable degrees and m-topped degrees

In this paper we will discuss recent work of the authors (Downey, Greenberg and Weber [8] and Downey and Greenberg [6, 7]) devoted to understanding some new naturally definable degree classes which capture the dynamics of various natural constructions arising from disparate areas of classical computability theory. It is quite rare in computability theory to find a single class of degrees which capture precisely the underlying dynamics of a wide class of apparently similar constructions, demonstrating that they all give the same class of degrees. A good example of this phenomenon is work pioneered by Martin [22] who identified the high c.e. degrees as the ones arising from dense simple, maximal, hh-simple and other similar kinds of c.e. sets constructions. Another example would be the example of the promptly simple degrees by Ambos-Spies, Jockusch, Shore and Soare [2]. Another more recent example of current great interest is the class of K-trivial reals of Downey, Hirscheldt, Nies and Stephan [5], and Nies [23, 24]. We remark that in each case the clarification of the relevant degree class has lead to significant advances in our basic understanding of the c.e. degrees. We believe the results we mention in the present paper fall into this category. Our results were inspired by another such example, the array computable degrees introduced by Downey, Jockusch and Stob [10, 11]. This class was introduced by those authors to explain a number of natural “multiple permitting” arguments in computability theory. The reader should recall that a degree a is called array noncomputable iff for all functions f ≤wtt ∅ ′ there is a a function g computable from a such that ∃x (g(x) > f(x)).